Historically, electric induction motors were designed to be operated on 3-phase sine wave power. This type of input power is termed balanced. When properly balanced, the common mode voltage – the sum of the 3 phases – equals zero volts and electrical bearing protection is generally not needed (except for very large motors, over 500 frame).
But when motors are operated by variable frequency drives (VFDs), the insulated-gate bipolar transistors used in most VFDs convert pure sine wave power to a series of positive and negative pulses. Consequently, input voltage to the motor is never balanced. Instead of a steady zero volts, it switches rapidly from positive to zero to negative and back, and the common mode voltage is usually a “square wave” or a “6-step” voltage waveform.
This nonzero common mode voltage in a motor’s stator windings causes motor bearings to “charge up” like a capacitor. For example, suppose that at one moment, the common mode voltage is positive inside the stator windings. This positive common mode voltage induces a positive voltage on the rotor through capacitive coupling. The motor frame is grounded at a neutral earth ground reference.
With the rotor now positive and the frame neutral, there is a potential difference across the motor’s bearings between the rotating shaft and motor’s grounded frame. Because the bearings are full of electrically insulating (dielectric) grease, which forms a non-conductive film between the balls and race walls, electrical current does not normally flow through the bearings. This works exactly like a capacitor: two conductors separated by a non-conductive dielectric. If you apply a voltage to a capacitor, it will “charge up,” and store energy, but not let any current flow through… unless the dielectric breaks down.
Every capacitor has a breakdown voltage*; the voltage above which the insulator breaks down and allows a burst of current to flow all at once. This is just like lightning: When the voltage between a cloud and the ground gets big enough, the air – normally a good insulator – becomes ionized and suddenly becomes conductive. The result is a rapid discharge: charge flows rapidly through the now-conductive air with the often-destructive release of stored energy. After discharge, the voltage across the capacitor will be zero (or at least much lower).
Now back to the motor: If the shaft voltage builds up high enough, it will discharge by arcing through the bearing: Electrons will leap through the bearing like a miniature lightning bolt, and destructively release the energy that was stored in the bearing’s “capacitance.” This arc is called a capacitive discharge current, and the damage it does to bearing surfaces is called EDM (electrical discharge machining). Capacitive EDM currents can occur in all motors run on VFDs, regardless of their size.
A second form of bearing current produced by VFDs — high frequency (HF) circulating currents — results from a high-frequency magnetic flux produced by common mode current (the sum of the currents flowing in each of the three phases). HF circulating currents consist of electrons arcing from the shaft through one bearing, running down the frame, and arcing through the other bearing back to the shaft. The frequency range for these circulating currents is in the kHz or MHz. The size of these currents, and the damage they do, depends on motor size. They first become a problem in motors above 100 HP (75 kW), and in general, the larger the motor, the greater the damage they cause.
Both capacitive discharge currents and high-frequency circulating currents damage the bearings. Because the bearing lubricant is dielectric, these currents can only flow by arcing through the grease. This both degrades the grease, reducing its lubricating properties, and damages the metal surfaces inside the bearing. This surface damage is directly caused by the electrical arcing, like tiny lightning bolts blasting through the bearing… thousands of times per second! Each arc creates a pit, a tiny molten crater where it strikes. As the pits accumulate, they lead to visible frosting, and eventually to fluting (washboard-like ridges in the bearing race). As bearings degrade, friction increases, leading to noise, deterioration of bearing grease/lubrication, and the likelihood of total bearing failure and costly unplanned downtime. In fact, this entire process can take place in as little as 3 months!
To learn how to prevent this electrical bearing damage from VFDs, read the companion article, Bearing Protection Best Practices
*For a bearing, the exact breakdown voltage will depend on RPM, temperature, loading, state of lubrication, state of wear, and many other variables. In practice, a bearing’s breakdown voltage is unpredictable, but measured shaft voltages under up to 5V peak (10V peak-peak) are considered safe, and higher shaft voltages will cause discharge.